1. Field of the Invention
The present invention generally relates to compound semiconductor devices and methods of fabricating the same and, more particularly, is directed to a compound semiconductor device constituted by a compound semiconductor including phosphorus (P) and a method of fabricating the same.
2. Description of the Prior Art
In a compound semiconductor device, for example, a compound semiconductor laser device, a so-called buried hetero (BH) type structure is required in order to decrease its threshold current in which an index of refraction is differed to both a lateral direction (direction in parallel to a cleavage plane) and a vertical direction (direction perpendicular to the cleavage plane) and a current limiting portion is provided to thereby confine carriers in the vicinity of a light emission region. The BH type semiconductor laser device can be fabricated mainly by the following two methods, for example. In one method, after a first cladding layer, an active layer and a second cladding layer are sequentially formed by the crystal growth process, then the second cladding layer and the active layer are partially etched away to the first cladding layer to form a so-called ridge structure, then the ridge is buried by a buried layer having a bandgap larger than that of the active layer which is formed by the crystal growth process to thereby confine carriers as to the lateral direction. In another method, after the respective layers are formed, then impurities are introduced by the ion implantation technique. These methods, however, have disadvantage such that the top surface of the layers contacts the atmosphere in the etching process, so that the crystal surface is polluted or the like to thereby degrade the characteristics of the device.
Another BH type semiconductor laser device has been proposed by Japanese Patent Laid-Open Publication No. 2-65288 filed by the same assignee as the present application and "Electronic Letters, 24 (1988) 1249" by H. Narui, T. Ohata and Y. Mori in which a step portion, that is, a ridge or groove is formed on a substrate such that orientation of its crystal Surface and the extended direction of the step portion or the like are selected suitably, then hetero junction boundaries are formed on the ridge so as to extend to the vertical and lateral directions by one crystal growth process. In this case, the above described BH type structure is obtained in a manner that a compound semiconductor constituted by AlGaAs based material is used as the substrate, and respective layers are sequentially formed by utilizing the metal organic chemical vapor deposition (MOCVD) technique on a major surface of the substrate which is constituted by a {100} crystal plane, to thereby form a discontinuous dislocation in the lateral direction of the semiconductor layer by utilizing a difference in growing speeds depending on crystal orientations.
However, in the semiconductor laser device constituted by compound semiconductor material, in particular, material including phosphorus such as AlGaInP based material or the like, since the growing speed thereof depending on the crystal orientation differs from that of the AlGaAs based compound semiconductor, it is difficult to use the above-described construction having the discontinuous dislocation.
According to the inspection by the inventors of the present invention etc., in a case of forming a step portion extending to the &lt;110&gt; crystal axis direction on a major surface constituted by the {001} crystal plane of a compound semiconductor substrate and then forming a compound semiconductor layer constituted by material including phosphorus (P) such as AlGaInP based material by the epitaxial growth process, it has been found by analyzing the structure of the AlGaInP crystal by a transmission electron microscope that the surface of the grown film is rough and so it is difficult to form a crystal plane with a good film quality or a smooth surface. Further, it has been found that the grown AlGaInP crystal includes many twin crystals formed therein and is not a single crystal.
This is seemed to be due to the following reason. That is, when the substrate with an accurate {001} crystal plane is used, a surface of the compound semiconductor layer formed on the slanted surface of the step portion becomes an accurate {111} B crystal plane. According to the inspection by the inventors of the present invention etc., however, it has been found that since twin crystals are formed on the surface of the accurate {111} B crystal plane, the surface of the compound semiconductor layer is degraded in film surface quality as well as crystal property, as described in "Journal of Crystal Growth, 106 (1990) 197" by E. Morita, M. Ikeda, M. Inoue and K. Kaneko. Thus, the reason why the surface of the grown film formed on the step portion of the substrate becomes rough seems to be mainly due to a fact that the accurate {111} B crystal plane is formed on the step portion.